As numpy is Python 3 only, these import statements are now unnecessary
and don't alter runtime behavior.

Add `print_function` to all `from __future__ import ...` statements
and use the python3 print function syntax everywhere.

Closes #3078.

This should be harmless, as we already are division clean. However,
placement of this import takes some care. In the future a script
can be used to append new features without worry, at least until
such time as it exceeds a single line. Having that ability will
make it easier to deal with absolute imports and printing updates.

Wraps the SetHook and calls to the hook with the GIL, to prevent races.
Adds an example of using the interface for callbacks into python code.